I would think the most important factor is the current owner. Is he/she the kind of person that would be very particular about filtering their wvo or a fly by the seat of your pants throw it in the tank type? Check out their filtering set up to see what you're getting into in regards to space, equipment and time. How mechanically able are you. Any modifications may require more upkeep and bringing a vehicle in can out weigh the savings pretty quick.

I looked at wvo pretty seriously but decided to go the wmo route instead. One tank no switching simpler system.

WMO= waste motor oil. I usually use waste auto transmission fluid I get for free and mix it 50/50 with diesel, after settling and filtering. I have managed to score some home heating oil on occasion which is diesel with red dye in it. Last year I picked up 100 liters of kerosene for free, that was nice. Your diesel will gladly run on motor oil, power steering fluid, tranny fluid and hydraulic fluid. I check my local transfer station and get a bit of clean stuff as well.